Such heaters are widely used in dishwashers, coffee percolators and other apparatuses where liquids are heated. They are mainly heated by tubular heaters, which are helically soldered onto the outside thereof (DE-32 21 348 C2). Attempts have already been made to work with rectangular heating resistors, whose insulation with respect to the heater wall is provided by a metal oxide coating on the same or on the wall of the container (DE-1 690 677 A1).
Attempts have also been made in the case of a continuous heater to apply heating conductors with a rectangular cross-section in the interior of the flown through tube to a plastic inner tube (DE-22 33 503 A1). In this case the liquid is under the operating voltage, which is usually inadmissible.
It can be gathered from all these earlier attempts that a main problem is the interposing of an electrical insulation between the heating conductor and the heater wall. Attempts have been made to keep this small through a relatively unreliable metal oxide insulation or to completely omit it when the heating conductor is located on the inside. Although this problem is less with tubular heaters, because the highly compressed embedding material of the tubular heater has a relatively good heat conducting action, the tubular heater and its soldering lead to relatively high manufacturing costs. In addition, with all the earlier flow heaters problems are encountered in fitting an excess temperature protection device and the response behaviour thereof is not good, unless extra costs are involved.